Hello everyone! That will be my first topic here
Few months ago I bought a broken Fluke 8840A in good (visual) condition for ~$50. Internal condition was unknown at the time. So... disassembly time! First impression was great but after some time I realised that the construction of 8840A is a nightmare. In order to remove the display and keypad I had to remove every PCB and cable...
1) Main board looks OK. No damaged capacitor, no visible short circuits, all ICs in place
2) TrueRMS & IEEE488 addons in great condition
3) Testpoints voltage in spec
4) VFD display... broken
After short research I have found Chinese project with LED replacement for VFD -
http://bbs.38hot.net/forum.php?mod=viewthread&tid=828474. Unfortunately there was no schematic or gerber files. In addition I didn't like the way they changed display layout. The decision was made to build my own LED display!
Requirements:
-Only discrete components allowed - not ICs
-Mimic the look of the original VFD display
-Low cost (as always)
Original driver outputs 0-30V signals that drive VFD anodes. High signal (30V) is used to drive both digit selector and segments. I used that fact in my advantage and with 1:2 resistor dividers NMOSFETs act as high and low switches for LEDs.
DC powers supply is generated by rectifier + capacitor powered from cathode heating voltage (~5V AC).
In version 1 I accidentally mirrored one 4 digit LED and half of the transistor but after many wire fixes it worked. That allowed me to perform an autotest on 8840A that succeeded without any error
Final version 2 got an additional PCB with “text mask” and blak 3d printed “light mask”. Thanks to that single LED lights up only one label text.
Finally I ended up with perfectly working Fluke 8840A for under $100. Now I need some references to make sure that calibration is not necessary. Only thing left to do is to replace old capacitors and find another broken device to repair.
[EDIT]
Gerbers added
Main board MaskBOM:
- All LED resistors near MOSFETs are 0402 10k
- All other resistors = 0402 47Ohm
- All MOSFETs = NMOS DMN6075S or similar
- C500 is used to lower rectifier ripples and anything at least 1000uF works
- D500, D502, D503, D504 - BAS3010B03WE6327HTSA1 - any SOD323 Schottky with at least 1A is OK